That can be tough. Not everyone is looking towards getting the same certification, even within the same company - or even the same technology stack. I lean to the Micro$soft stack and, from experience, there are a plethora of certifications even within the developer arena. However, let's assume (for the sake of discussion) that you have a group who want to get the same cert.
We had a group set up at work...I would not make this group much larger than ~4 or else you get too much "noise" and other distractions. We agreed to meet once per week and kept each other accountable for studying the assigned material. We all ordered the same certification study book(s) and would go over the lessons learned from the previous week and then an overview of the expectations for the next week. We met for an hour over lunch - and of course, there were always the impromptu discussions that occurred through the week when someone had questions, or if something applied to our current task.
I think it was optimal for when you are at work together. However, it can work for those who do not work in the same office as well. If you don't have a couple of others in the office interested in getting certified, then you should be able to get involved with a local users group. In those cases, you may have to meet before work or meet during lunch - logistically a bit more difficult, but definitely do-able - AND you make some great technical contacts - networking is key.
Just my 2 cents - good luck!!